Enclosing semiconductor chips with a resin material by transfer molding is well known.
With reference to FIG. 11 showing this process, a lead frame D for semiconductor chips G is set in a cavity C formed between an upper die A and a lower die B. A molten resin material E is then injected into the cavity C through a gate F to enclose the chips G on the lead frame D with the resin filling the cavity C.
An air vent H is provided in the plane of a parting line PL between the upper and lower dies A and B for the cavity C to communicate with the outside therethrough. Accordingly, the air I within the cavity C is forced out through the air vent H by the molten resin material E injected into the cavity C.
The air I is thus completely discharged from the cavity C through the air vent H so that the surface or interior of the resin body molded in the cavity C will be free of voids that would be formed if some air remains in the cavity C. The presence of the air vent H is therefore one of the requirements for producing resin-enclosed semiconductor chips which must be highly resistant to moisture and reliable.
The air I is discharged through the air vent H by being forced out by the molten resin material E injected into the cavity C, with the result that a portion of the resin material E fills the air vent H to create the following objections.
Since the air vent H is provided by a clearance formed in the plane of the parting line PL, the resin material solidified in the air vent H is liable to adhere to and remain on the opposed surfaces of the dies A and B to impair the function of the air vent. Further in the case where the molding apparatus is large-sized, an air vent is formed in the surface of each of the upper and lower dies, so that the solidified resin is difficult to remove. Consequently, the dies are used for the subsequent molding operation without completely removing the solid resin from the air vents. If the air vent H is completely or partly closed with the solid resin, it becomes impossible or difficult to discharge air from the cavity C, permitting some air to remain in the cavity C. It is then impossible to completely prevent formation of voids in the surface or interior of the molded resin body.
The surfaces of the dies A and B are usually cleaned manually for the removal of solid resin. The cleaning operation requires a cumbersome procedure if it is to be conducted completely. Further the cleaning operation, which needs to be conducted on completion of every molding cycle, results in a greatly reduced molding operation efficiency. Further because the cleaning operation is conducted with the two dies A and B moved away from each other upward and downward, the operation is hazardous, and the time-consuming cleaning work is not desirable in assuring the operator of safety.